Usually, a maintenance hole or the like in a road surface consists of an underground vault or chimney that supports on its upper end a rigid frame, usually of cast iron, the upper edge of which is desirably flush with the road surface. The rigid frame is apertured and supports a cover such as a maintenance hole cover, valve chamber cover, catch basin grating, or the like. In conventional frame and cover assemblies, the side wall of the frame is provided at its lower edge with an outwardly directed flange to assist in maintaining the position of the frame in the road surface. When the frame is installed in the road surface with its upper surface approximately flush with the road surface, the flange underlies the asphalt about the perimeter of the frame.
It has been found that the road surface adjacent the perimeter of the frame is particularly prone to deterioration, cracking and fragmentation, necessitating frequent repairs. The primary reason for the rapid deterioration of the road surface in the vicinity of the frame is that, when vehicles move over the frame and cover assembly, a certain amount of vibration is transferred from the moving load of the vehicle through the frame and cover to the asphalt adjoining the frame. It has been found that vibrations from moving vehicles are largely transferred to the asphalt by the flange at the base of the frame. While new asphalt tends to be flexible and able to absorb such vibrations, older, more brittle asphalt will tend to break up, particularly under cold winter temperatures.
Conventional frame and cover assemblies are usually also provided with means by which the cover may be secured to the frame, for example to prevent children from removing the cover and falling into a sewer. For example, in one prior art device shown in Ontario Provincial Standard Drawing OPSD-401.050, the frame is provided with a series of apertured lugs extending inwardly into the opening of the frame. The cover is bolted to these lugs through holes provided inwardly of the edge of the cover. It has been found that the provision of inwardly directed lugs on the frame restricts access of personnel or equipment through the opening. To date, no completely satisfactory means has been developed for securing the cover to the frame.